Test (biology)

A test is a rigid outer covering found in a variety of marine and freshwater organisms. It is distinct from a simple cell wall or a soft cuticle, being typically mineralized and often intricately structured. The primary functions are protection from predators and environmental stress, support for soft tissues, and, in some cases, buoyancy control.¹

Tests are characteristic of several groups: Foraminifera (protists with calcareous or agglutinated tests), Radiolaria (siliceous tests), Testate amoebae (proteinaceous or siliceous tests), Sea urchins (calcareous endoskeleton), and some algae like Acetabularia (calcified).

Tests are composed of various materials: calcium carbonate (calcite or aragonite) in foraminifera and echinoderms, silica in radiolarians and diatoms, and organic compounds in some testate amoebae. Formation occurs through biomineralization, where the organism secretes minerals in a controlled manner, often using organic matrices as templates.

Because tests are durable, they are abundant in the fossil record. Foraminifera tests are used extensively in biostratigraphy to date sedimentary rocks and in paleoclimatology to reconstruct past ocean conditions via oxygen isotope analysis. Radiolarian tests are similarly important for dating deep-sea sediments.²

Beyond the well-known groups, tests are found in some lesser-known organisms: tintinnids (ciliated protozoans with loricae), pteropods (sea snails with delicate aragonite shells), and brachiopods (lamp shells). Some foraminifera build tests by agglutinating sediment grains, a process called agglutination. The largest foraminiferan, Nummulites, has tests up to 6 cm across. In sea urchins, the test is an endoskeleton made of fused plates, and its growth involves resorption and regeneration. The study of test microstructure has inspired biomimetic materials research.

Biomineralization

The process by which living organisms produce minerals, often to form hard structures like tests.

Agglutinated test

A test constructed by cementing foreign particles (e.g., sand grains) together.

Lorica

A protective shell or case in some protozoans, analogous to a test.